cervical length

i'm a bit confused. last week - at 19wks pregnant i had a transvaginal ultrasound that said my cervix length was 3.4cm or 34mm and everything over 3cm is acceptable.

however, when i read charts and stats i should be closer to 5cm

so having had a preemie already i am a bit concerned

not flat out worried but if you read this chart maybe you'd see where i might be concerned